1. Field of the Invention
The present invention relates to diagnostic ultrasound imaging by which imaging is performed based on a contrast echo technique for an object into which an ultrasound contrast agent of which main constituent is microbubbles is administered. In particular, the present invention relates to diagnostic ultrasound imaging capable of separating transient signals caused due to the microbubbles of the ultrasound contrast agent on the basis of a rate subtraction imaging (RSI) technique.
2. Description of the Related Art
Ultrasound signals have been clinically used in various fields, and one field is an application to diagnostic ultrasound apparatus. A diagnostic ultrasound apparatus acquirers an image signal through transmission and reception of an ultrasound signal toward and from an object and is used in a variety of modes utilizing non-invasiveness of the signal.
One typical type of diagnostic ultrasound apparatus produces tomographic images of a soft tissue of a living body by adopting ultrasound pulse reflection imaging. This imaging method is noninvasive and produces tomographic images of the tissue. Compared with other medical modalities such as diagnostic X-ray imaging, X-ray CT imaging, MRI, or diagnostic nuclear medicine imaging, this pulse reflection imaging has many advantages: real-time display is possible, a compact and relatively inexpensive apparatus can be constructed, patient""s exposure to X-rays or others will not occur, and blood imaging is possible thanks to ultrasound Doppler imaging. This imaging is therefore widely used for diagnosis of the heart, abdomen, mammary gland, and urinary organs, and for diagnosis in obstetrics and gynecology. In particular, pulsation of the heart or motion of a fetus can be observed in real time through manipulation that is as simple as just placing an ultrasound probe on a patient""s body surface. Moreover, since patient""s X-ray exposure need not be cared about, screening can be carried out many times repeatedly. There is also the advantage that the apparatus can be moved to a bedside position for ready screening.
In the field of the ultrasound diagnosis, a trans-venous injection type of ultrasound contrast agent has been commercially available. This agent, which is injected into an object through the vein in screening the heart or organs in the abdomen, is used to enhance echo signals emanated from flows of blood so as to evaluate blood flow kinetics. This imaging is known as contrast echo imaging. Since the trans-venous injection of the contrast agent is less expensive than its trans-arterial injection (i.e., trans-arterial injection type of contrast echo imaging), diagnosis based on the trans-venous injection has been spotlighted. The main constituent of the contrast agent is microbubbles that act as sources to reflect ultrasound waves. The larger the amount or concentration of an injected contrast agent is, the larger the contrast effect is. However, the microbubbles are delicate substances, so they are characteristic of collapse due to irradiated ultrasound waves. It has been found that various conditions, such as, an extremely shortened duration of the contrast effect resulted from a certain condition of ultrasound waves, will occur. Although a contrast agent of high persistency and high durability has been developed recently, the collapse phenomenon of the contrast agent cannot be avoided fundamentally, because its main constituent is microbubbles. Meanwhile, the long-term persistence of the contrast agent in a human body may raise the problem of invasiveness.
In the contrast echo imaging, a contrast agent (i.e., microbubbles) is successively supplied into a region of interest of an object through blood flow. Hence, it is assumed that, even when once bubbles existing within the region collapsed by irradiated ultrasound waves, the contrast effect will be maintained as long as new microbubbles will inflow into the region of interest at the next timing of ultrasound irradiation. However, ultrasound waves are normally transmitted and received as many times as a few thousands per second, there is organ parenchyma whose blood flow speed is rather slow, and there is blood kinetics in relatively thin blood vessels. Considering these conditions, microbubbles will collapse in turn before observing on a diagnostic image intensified intensity of data due to a contrast agent, thus the contrast effect being lessened instantaneously. Various reports concerning this collapse phenomenon of microbubbles have already been published and flush echo imaging (FEI) which will be described later belongs to the imaging techniques based on the collapse phenomenon.
Of diagnostic techniques using the contrast agent, the most fundamental diagnostic technique is to detect whether or not there is blood flow in a region to be diagnosed by examining the existence of intensified intensity data depending on the contrast agent. More advanced diagnostic techniques include a technique of detecting temporal changes in spatial distributions of the contrast agent from spreads of changes in intensity or from degrees of intensified intensity data in a diagnostic region. Also included is a technique of acquiring an interval from the start of injection of a contrast agent to its arrival at a region of interest (ROI) and temporal changes in intensity data (Time Intensity Curve: TIC) or a maximum of intensity data, both of which is due to the contrast agent, within the ROI.
The contrast echo imaging can also be performed effectively with harmonic imaging (HI) using a non-fundamental component of ultrasound waves. The harmonic imaging is based on separation and detection of only a non-fundamental component derived due to nonlinear behaviors of ultrasound-excited microbubbles, which are main constituents of a contrast agent. Since internal organs of a living body are relatively difficult to cause nonlinear behaviors, the harmonic imaging can give contrast agent images with preferable contrast ratios.
It has been known that echo signals emanated from tissue such as internal organs include non-fundamental components (mainly, harmonic components), though it is lower in level than that emanated from the contrast agent, and the non-fundamental components are mixed with the entire received echo signal. Such harmonics from tissue are normally called as tissue harmonic signals, which give a basis to imaging, called tissue harmonic imaging (THI).
As practical techniques of detecting and imaging non-linear signals (mainly harmonic signals), there have been known two techniques. One technique is that a high-pass type of echo filter is used to cut off a fundamental component, providing only a filtered harmonic component used for recombining images. The other is that a technique referred to as an inverted phase pulse adding technique (pulse inversion technique) is used to extract a harmonic component that is involved in recombining images.
Of these techniques, the inverted phase pulse adding technique is disclosed, for example, by U.S. Pat. No. 5,632,277. Practically, in this imaging, two ultrasound pulses of which phase difference is 180 degrees from each other are transmitted along each raster (two times of transmission in total), resultant echo signals are received, respectively, and then the echo signals are added to produce added signals for imaging. This addition causes linear components included in the mutual echo signals to be cancelled out, because one of the echo signals is inversed in the waveform and both echo signals are added. Accordingly, a doubled harmonic component remains (180 degreesxc3x972=360 degrees). This technique requires two pulses to be transmitted and received for each raster, thus a frame rate is reduced. However, design of filters is easy, even if the high-pass type of filter is used. In addition, a transmission pulse of which bandwidth is wide can be used, increasing spatial resolution.
By the way, it has been reported that the phenomenon that the microbubbles are vanished by irradiating ultrasound waves is used for imaging called flash echo imaging (also called a transient response imaging), increasing enhancement of intensity (for instance, refer to Japanese Patent Laid-open Publication No. 8-280674). This imaging is based on the principle that, instead of the conventional continuous scan carried out at a rate of tens of frames per second, intermittent transmission is performed at a rate of one frame per a few seconds. During each interval between the two times of transmission, microbubbles are made to gather into a region to be scanned without collapses. The gathered microbubbles are then vanished at a time by the next transmission, generating an echo signal of higher intensity.
The foregoing harmonic imaging and the flash echo imaging are not conflicting techniques, but can be combined to be used together. These two imaging techniques can be classified from a viewpoint of usage as below. As to the harmonic imaging, the techniques of
a) fundamental wave imaging
b) harmonic imaging on filters, and
c) inverted phase pulse adding technique
belong to the same category. Any one technique is selected from the above imaging types a) to c) and used. On the other hand, as to the flash echo imaging, the techniques of
axe2x80x2) continuous transmission
bxe2x80x2) flash echo imaging (that is, intermittent transmission) are provided, in which either technique axe2x80x2) or bxe2x80x2) are chosen. Alternatively, any of the former techniques a) to c) and either technique axe2x80x2) or bxe2x80x2) can be used together.
As described before, use of a trans-venous type of contrast agent and use of imaging, such as the harmonic imaging or flash echo imaging, in an appropriate mode make it possible to image minute blood flow, i.e., perfusion, in the tissue of an internal organ.
However, it is not always possible for the conventional imaging to provide images of the perfusion with stability, there frequently occur the following problems.
One problem is that a diagnostic performance lowers due to a tissue harmonic signal.
A tissue harmonic component generated from the tissue of an internal organ is fundamentally lower in intensity than that from a contrast agent, as described before. Actually, however, there occur situations that make blood flow diagnosis difficult. The tissue harmonic signal differs individually and there are many persons that generate echo signals of relatively larger intensities. From a viewpoint of experience, for diagnosing the liver, for example, there is known that intensities of echo signals are different largely between a region with a lesion such as hepatocirrhosis or a fatty liver and a region with no such lesion. Since the tissue harmonic signal is caused resultantly from a non-linear characteristic of transmission of ultrasound waves, it increases as the sound pressure (output) of a transmitted ultrasound pulse is raised. In contrast, an echo signal reflected by the contrast agent is apt to instantaneously vanish when the sound pressure of the transmitted ultrasound pulse is over a given value. As a result, there occurs a problem that echo intensities of an internal organ itself is raised before injecting a contrast agent. If this occurs, it is difficult to confirm whether or not echoes around thin blood vessels on images (particularly, a region with minute blood flows) acquired after the injection of the contrast agent are resulted from the contrast agent. This problem is also true of the inverted phase pulse adding technique.
A second problem concerns with optimization for generation of images and processing of display.
The steps of recombining echo signals acquired on each raster into a tomographic image and displaying the image are involved with a variety of types of processing. Conventionally, those types of processing are optimized depending on which region is diagnosed. For example, for diagnosing an internal organ of which motions are relatively slow, such as the liver, a high frame rate is not necessary, so a density of rasters are increased to raise spatial resolution. From the same reason, after-images of several past frame images are added to suppress flickers appearing on images on account of noise or others. In the case of the heart, images are subjected to spatial differentiation of their intensities in order to emphasize the boundaries of the heart walls. Processing to enhance the depiction performance of the endocardium and epicardium is carried out as well.
However, when depicting echo signals from microbubbles of a contrast agent, it is not necessarily true that the foregoing tissue depiction processing is suitable for the depiction of such echo signals and they are depicted in an optimum state. Gathered microbubbles may show tiny speckle patterns, in which the collapses of the microbubbles are also caused. Accordingly, there are some occasions in which the contrast agent, that is, minute blood flows are seen on and off with motions. By this appearance, intensity is increased due to the contrast agent and visibility is improved due to the speckle patterns. In such situations, however, if the foregoing frame after-image processing and/or differentiation are carried out, the visibility for echo signals from the contrast agent is decreased.
To overcome this problem, in the conventionally performed contrast echo imaging, conditions for producing images giving priority to behaviors of microbubbles are frequently determined based on experienced knowledge. However, in such cases, there occurs the problem that the depiction of tissue of an internal organ is largely deviated from its optimum state so that its depiction performance is remarkably deteriorated.
The present invention has been made to overcome the above problems. An object of the present invention is to provide ultrasound imaging, in which, for performing a contrast echo imaging under the injection of a contrast agent into an object to be screened, the contrast agent, i.e., minute blood flows is distinguished from the surrounding tissue with precision so as to increase a depiction performance, and the contrast agent and tissue are individually imaged in their optimum conditions so as to increase image quality, thereby improving a diagnostic performance for the minute blood flows.
In order to realize the foregoing object, a diagnostic ultrasound apparatus according to the present invention uses rate subtraction imaging (RSI) that requires subtraction raster by raster.
According to a practical configuration of one aspect of the present invention, the diagnostic ultrasound apparatus for obtaining an image of a region to be scanned by scanning an object with a beam-shaped ultrasound signal, an ultrasound contrast agent (of which main constituent is microbubbles, for example) being injected into the object, comprising: scanning means for transmitting the ultrasound signal a plurality of times in each direction composing the region to be scanned and receiving an echo signal in response to each transmission; subtracting means for obtaining a difference signal by performing subtraction between the echo signals received with two times of transmission among the plurality of times of transmission; producing mean for independently producing both of the echo signal received with any time of transmission of the plurality of times of transmission and the difference signal into individual tomographic images; and displaying means for displaying the individual tomographic images at the same time.
For example, the plurality of times of transmission is two times of transmission in each raster composing each direction. Preferably, the producing means has processing means for processing any of a first echo signal and a second echo signal received with the two times of transmission and the difference signal under processing conditions mutually independent of each other into data of two tomographic images. By way of example, the processing conditions include at least one of a reception gain, a dynamic range, cut-off frequency and bandwidth of an echo filter, and a frame-to-frame after-image processing. Further, the processing conditions may include at least one of a correction processing technique and a color encoding technique performed when mapping intensities of the echo signal on a video screen.
Furthermore, the displaying means may have means for displaying the individual tomographic images in mutually different colors.
Further, for example, the displaying means is means for displaying the individual tomographic images with either one tomographic image superposed on the other tomographic image. In this situation, displayed is the tomographic image based on the difference signal, which serves as the other tomographic image, superposed on the tomographic image based on the echo signal, which servers as the one tomographic image. Preferably, of the one and other images, either one image is displayed in gray scales and the remaining image is displayed in colors. Another preferred example is that the displaying means is means for displaying the one and other images in mutually different colors. Further, the displaying means may be means for displaying the individual tomographic images in parallel with each other on a screen.
Accordingly, for example, during a scan, a transient echo component inherent to a contrast agent is separated and extracted from an original echo signal as a difference signal based on the rate subtraction imaging. And the difference signal and the original echo signal are processed independently of each other into separate tomographic images. The tomographic images thus processed are then displayed at the same time. In other words, the transient contrast echo image and the steady tissue echo image are processed in their optimum states, respectively, and then displayed together, for example, in a superposition display mode or a parallel display mode.
Therefore, there is a fundamental difference in signal processing between the present invention and either of 1) the conventional harmonic imaging or 2) the conventional pulse inversion imaging; in 1) the conventional harmonic imaging, one type of echo signal in which a fundamental wave and non-fundamental waves are mixed together is processed through from a probe to a display, and in 2) the conventional pulse inversion imaging, two echo signals from two times of transmission are combined before a receiver, then one combined signal is processed for display. In contrast, 3) the signal processing in the present invention is equivalent to the technique that echo signals inherent to tissue and a contrast agent are subject to processing from a probe to a frame memory (data synthesizer) independently of each other, and then combined at the stage of being displayed.
It is therefore possible to not only improve both images of the contrast agent echo and tissue echo but also preventing steadily minute blood flows in tissue from being hidden by tissue echoes so that visibility is lowered. Additionally, for observing dynamics of minute blood flows in tissue under the contrast echo imaging, a tissue echo image is always provided together with a contrast echo image to be targeted. This makes easier to understand spatial locations of distributions of minute blood flows.
The imaging according to the present invention is not limited to that carried out during a scan, but can be attained after the scan (i.e., after diagnosis on the spot) through display using echo data stored after being subjected to the above processing.
In order to attain this imaging, by way of example, the producing means has image memorizing means capable of individually memorizing image data of the individual tomographic images and individually reading out the image data thereof. Preferably, the displaying means has means for individually reading out the image data of the tomographic images from the image memorizing means and means for commanding a switchover of display modes consisting of superposed display of the individual tomographic images, parallel display of the individual tomographic images, and sole display of either one of the individual tomographic images on the bases on the read-out image data. As an example, the displaying means include means for independently setting at least one of a correction processing technique and color encoding technique accordingly to which of the echo signal and the difference signal corresponds to the read-out image data and mapping the image data on a video screen.
According to another aspect of the diagnostic ultrasound apparatus of the present invention, in the apparatus configuration identical to the foregoing basic aspect, for the echo signal, there is provided addition means for obtaining an average signal by performing average between the echo signals received with two times of transmission among the plurality of times of transmission, and both of the difference signal and the average signal produced into individual tomographic images, and the individual tomographic images are displayed at the same time.
In this case, preferably, the plurality of times of transmission is two times of transmission in each raster composing each direction. It is preferred that the producing means includes processing means for processing the average signal and the difference signal with processing conditions mutually independent of each other into data of two tomographic images. Preferably, the displaying means is means for displaying the individual tomographic images in either one of a superposition manner and a parallel manner.
As another aspect of the diagnostic ultrasound apparatus of the present invention, it may be configured in a such a manner that the subtraction means is means for performing the subtraction of the echo signal received by the first-time transmission of the plurality of times of transmission and the echo signal received by any-time transmission selected from the second-time or later transmission of the plurality of times of transmission, and the addition means is means for performing the addition of the echo signal received by the first-time transmission of the plurality of times of transmission and the echo signal received by any-time transmission selected from the second-time or later transmission of the plurality of times of transmission.
In the present invention, in the foregoing first and second aspects, it may be configured such that the scanning means is means for additionally transmitting an ultrasound signal to excite the microbubbles in each direction. In this case, it is preferred that the additional transmission of the exciting ultrasound signal interleaves in time between two times of transmission selected from the plurality of times of transmission.
Furthermore, in the present invention, in the foregoing first and second aspects, it is preferable that the displaying means has means for commanding a switchover of display modes consisting of superposed display of the individual tomographic images, parallel display of the individual tomographic images, and sole display of either one of the individual tomographic images.
On the other hand, according to one aspect of an ultrasound imaging method of the present invention, there are configurations comprising the steps of: transmitting the ultrasound signal a plurality of times in each direction composing the region to be scanned and receiving an echo signal in response to each transmission; obtaining a difference signal by performing subtraction of the echo signals received with two times of transmission among the plurality of times of transmission; independently producing both of the echo signal received with any time of transmission of the plurality of times of transmission and the difference signal into individual tomographic images; and displaying the individual tomographic images at the same time.
As another mode of the configuration of the ultrasound imaging method, there may be the additional steps of calculating an average signal by performing average between the echo signals received with two times of transmission among the plurality of times of transmission; independently producing both of the difference signal and the average signal into individual tomographic images; and displaying the individual tomographic images at the same time.
Still, another mode of the configuration of the diagnostic ultrasound apparatus is provided such that the diagnostic ultrasound apparatus for obtaining an image of a region to be scanned by scanning an object with a beam-shaped ultrasound signal, an ultrasound contrast agent being injected into the object: an ultrasound probe for transmitting and receiving the ultrasound signal; a transmitter for exciting the ultrasound probe responsively to each rate pulse so as to cause the ultrasound probe to output the ultrasound signal; a receiver for delaying and adding an echo signal received by the ultrasound probe; a controller for causing the transmitter to transmit the ultrasound signal a plurality of times in each direction composing the region to be scanned and causing the receiver to receive the echo signal in response to each transmission; a subtracter for obtaining a difference signal by performing subtraction between the echo signals received with two times of transmission among the plurality of times of transmission; a producer for independently producing both of the echo signal received with any time of transmission of the plurality of times of transmission and the difference signal into individual tomographic images; and a display for displaying the individual tomographic images at the same time.
Preferably, the plurality of times of transmission is two times of transmission in each raster composing each direction. By way of example, the producer has a processor for processing any of a first echo signal and a second echo signal received with the two times of transmission and the difference signal under processing conditions mutually independent of each other into data of two tomographic images.
Still preferably, the display is configured to display the individual tomographic images with either one tomographic image superposed on the other tomographic image. Particularly, it is preferred that the display is changeable, for example, by hand, in a superposition balance of intensity when the other tomographic image is superposed on the one tomographic image. Still preferably, the echo signal experiencing the subtraction executed by the subtracter is a radio frequency signal of the echo signal before detected.